It is frequently desirable to obtain high laser energies by amplifying a low energy laser oscillator. The laser oscillator design and construction are thereby simplified since the oscillator is not required to handle high energies.
The conventional design of the laser amplifier is to store energy in the amplifier medium such that a high population inversion is maintained for the lasing transition. The incident laser beam then dumps this population inversion and it is amplified as it traverses the amplifier.
However, a two-photon, three-level laser, such as the 16 .mu.m CO.sub.2 bending mode laser, lases at two wavelengths, 9.4 and 16 .mu.m, where the lasing involves three vibrational levels, and where the lower laser level for the first lasing transition is the upper laser level for the second lasing transition. More specifically, the first lasing transition dumps energy from the upper laser level to an intermediate laser level. A population inversion is thereby created between the intermediate laser level and the lower laser level which results in the second lasing transition.
Since the population inversion for the second lasing transition results from the first lasing transition, and is quickly depleted by collisional relaxation processes, an amplifier for a two-photon, three-level laser oscillator cannot store energy by maintaining a high population inversion for the second lasing transition.